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Adding a Soft Start to Water Pump Motors - Reducing Relay Burning Problems

In this post we discuss a few innovating and simple soft start circuit examples which may be implemented with heavy duty motors so that they are able to initiate with a soft start or a slow sluggish start instead of a sudden, bumpy start

Why Soft Start is Crucial for Heavy Motors

When heavy motor systems or high current motors are involved, initial switch ON current surge often becomes an issue. This surge tends to inflict huge arcing across the pump relay contacts causing corrosion and reduction in its life due to stress, and wear and tear.

The high current arcing not only causes relay contact issues, but also affects surrounding electronic circuits, causing them to hang or get disturbed due to large amount of RF interference generated during motor switch ON.

However safeguarding the costly motor relay becomes the main issue with such situations. Though there are many mechanical contactors available for controlling motor stress, these system are not efficient and are ineffective against the RF emissions.

The simple electronic circuit presented below hopefully is able to eliminate all issues concerned with heavy motor switch ON surge generation and relay contact protection.

The figure shows a simple dimmer switch circuit incorporating an ordinary triac and diac configuration, which can be very effectively used for adding a soft start to any high current, heavy AC motor.

Designing a Soft Start using Triac Phase Chopping

Here the control pot has been replaced with a LED/LDR box. As we know that in normal dimmer switches, a variable resistance is used for controlling the fan speeds. Here the variable resistance is replaced with a LED/LDR arrangement. It means now the speed of the motor, or in other words, current to the motor can be controlled by controlling the intensity of the enclosed LED through an external trigger.

That's exactly what is done here. When the motor relay is switched ON, either by a switch or through an electronic control circuit such as a water level controller circuit, the LED of the attached dimmer switch is also switched ON simultaneously.

The LED switches ON the triac and the connected motor.

Being a solid state device the dimmer switch acts a little faster than the relay and therefore the motor is first activated through the dimmer triac and just after a few milliseconds the triac gets bypassed by the concerned relay contacts.

The above process completely eliminates any sparking from the relay contact since the triac has already absorbed much of the current and the relay only has to softly takeover the already switched ON motor conduction.

Here the brightness of the opto-coupler LED is crucial, and must be set such that the triac is only 75% ON.

This adjustment will save the triac from initial heavy current transient and help the entire system to last for many many years.

The resistor R4 may be appropriately set for achieving an optimal glow over the LED.

Circuit Diagram

Parts List

R1 = 15K
R2 = 330K,
R3 = 10K,
Diac resistor = 100 Ohms,
R4 = to be adjusted as explained,
C1 = 0.1uF/400V
C2, C3 = 0.1uF/250V,
L1 = 10 amp/220V choke
Triac (Alternistor) = 10 Amp 400V,
Diac = as per the above triac.

Upgrading Triac Soft Start with Relay

soft start for motors with relay and triac

A little inspection reveals that the circuit actually does not require the opto coupler circuit at all. The circuit may be simply arranged in the following manner:

R2 should be selected such that the triac conducts only 75% of the power.

When power is switched ON,  the triac provides a soft initial start to the motor until within the next split second when the relay also conducts enabling the motor the required full power. This completely safeguards the actuator contacts from the initial current surges and sparks,

Simplified Soft Start Design

As rightly suggested by Mr.Jim, an initial torque is imperative for initiating a motor optimally especially when it's loaded, if this initial torque is absent. the motor might stall with heavy loads under its belt and might start smoking within minutes.

The following circuit is designed for solving both the issues together, it inhibits the initial surge current to the ON/OFF switch and yet allows the motor to start with a "kick" so that it initiates without problems even when it's loaded.

The above design can be even further simplified by removing the relay, as shown below:

An technicallu more sound PWM based motor soft start circuit can also be tried for getting a better control, a better torque and a reliable startup for the connected motor, even for 3 phase motors.

Soft Start Using Controlled Phase Chopping

Another way of implementing triacs through stepped phase chopping, for initiating slow soft start and slow end or slow stop circuit for heavy machine motors so that the motors are able to go through a gradually start stop actions instead of switching ON/OFF abruptly.

The idea is basically intended to ensure less wear and tear on the motor and additionally save electricity during the course of actions.

The idea was requested by Mr. Bernard Botte.

Dear mister Swagatam,
Sorry for my English , thanks anyway for any answer you will give Before the question. I use different apparatus to handle wood using universal AC motor originally made for a range between 230 to 240 volt 50hz (but I notice in certain part of my country 250V too) because I need a lot off different kind of machine and that was only for hobby.

I buy the cheapest machines I can find (I correct certain mechanical problems) for other machines. I use also a dimmer (home made based on the system used by vacuum cleaner and modified byNINA67 ) and It work great.

But I also use a planer/thicknesser using a motor rotating at 18000 T/min. It seems made to notpay any royalties to evict copyrights. Before I had problem I tought it was a motor running at 3000 t/min (2700) multiply by 2 (like others) with a belt to reach a decent speed of 6000 t/m (5400)Sorry no. And I don’t use the dimmer.

The motor run at +/- 18000 : 3 =6000 !!! Knowing the cheap cost of that machine I use it like a “good father” not intensively etcBut one day there was a fume

The machine smoke and i dismount the machine to isolate the motor to evict the fire . (the machine was under warranty but i need to make a lot off kilometers to make an exchange. And there, they don’t tell me it was a well known and reccurent problem … but … they know it! )

In fact when everything was cold . I look the axis who rotate he seems also shooting on the opposite side of the gear belt at every start Like there wasn’t a grower.

I show the motor in a company saling different kind of motor.

They make also refurbishing but they explain to me that it was an “exotic” motor but they set the same diagnostic .Start to fast So come my question: Could you please make a schematic to have a “soft start / soft ending ” for different universalmotors in fact if i use my dimmer system based on BTA 16 800 cw (better than the other mentioned above) it seems ok but i have only made 3 of them . I’ll want to integrate that in every big machine .

And use only the on/off switch.I want use thus a button to “switch on” and one to “switch off” or an on/off switch.

But also a potentiometer to select the minimum level (depending the power of each motors) when the motor start running and a potentiometer to select the timing (555) between the slow start and the full speed (maybe also shortcut the triac with a relay to have full speed an a green led if it is relevant (but it will be nice) for the switch off the timing maybe reduce. Why at the end because the extra current and problems binded.

Note : I have seen this application with “fpla” or dedicated processors but I am sure it can also be done with discrete components.Why i can not do that : because I never study the motors correctly but I know for example that it is not correct to start the motor with a zero crossing system because It give a maximum current and that make the same trouble (FIRE!) with the couple at start and max current …

I have seen this request in other forum touching other job mechanic wood etc … with no answer and people say also if it work with a potentiometer but when you change from a machine to another you can make mistakes etc…Regards Botte Bernard (Belgium)please don’t put my adress on the net Nb i like also in your presentation the datasheet because it’s no so easy to have it without paying

Bernard Botte

Designing the Stepped Phase Control Circuit

The requested idea of a soft start, soft stop motor switching circuit can be implemented using a simple triac based dimmer switch concept, as presented in the following diagrams:

Referring to the above diagrams, the first diagram shows a standard light dimmer or a fan dimmer switch circuit using a heavy duty triac BTA41A/600.

The section which indicates the “4 triac module” is normally occupied with a potentiometer for enabling a manual speed control adjustment, wherein a lower resistance adjustment generates higher speed on the fan motor and vice versa. In this soft start, soft stop design, this pot section is replaced with the indicated 4 triac module which can be elaborately visualized in the second diagram.

Here we see 4 triacs arranged in parallel having 4 individual 220K resistors at their upper MT1 arm, and 4 individual capacitors at their gates with different values, and with a sort of sequential order of high to low. When S1 is switched ON, the triac having the lowest value capacitor switches ON first, enabling a relatively slow speed start on the motor due to switching of the relevant 220K resistor at its MT1.

Within a few millseconds the next subsequent triac conducts which has the next smaller value, and adds its own 220K resistor in parallel with the earlier 220K resistor, allowing the motor to gain some more speed. Identically, the third and the fourth triacs also sequentially switch ON within the next few milliseconds, thereby adding two more 220K parallel resistors in the range, which finally allows the motor to reach its maximum speed.

The above sequential speed increase on the motor allows the motor to achieve the intended slow start switch ON, as desired by the user.

Quite similarly when the switch S1 is turned OFF, the relevant capacitors switch OFF in the same order but in a descending manner, which inhibits the motor from a sudden stop, instead it causes a step wise slow stop or slow end on its speed.

Feedback from Mr. Bernard:

Dear mister Swag, First of all, thanks for your fast answer. Because you tell me you have a timing problem I have changed my operating system to linux mint 18,1 ‘Serena’ so i haved to re-install all the program I need and test it (setup it!) So apparently everithings seems working OK ! About the first schematic I notice you don’t give any value to the upper side schematics so I pick it up from « How to Make a Simplest Triac Dimmer Switch Circuit »

Parts List for the above enhanced fan dimmer circuit(C1) C7 = 0.1u/400V
(C2, C3) C8,C9 = 0.022/250V,
(R1) R9 = 15K,
(R2) R10 = 330K,
(R3) R11 = 33K,
(R4) R12 = 100 Ohms,VR1 = 220K, or 470K linear => Replaced by genial 4 triacs module
Diac = DB3,
Triac = BT136 => BTA41 600
L1 = 40uH

About the second schematic so simple solution i never have dreamed !!! to be tested asap Genial! we say in French.

I doesn’t know that you can use polarized condensators for such AC applications! And also that 50 volt was sufficient! I you have a moment to explain why -

Anyway maybe i’ll try it this weekend if i have all the component. I prefer use new capacitors my stock never change since 1993!

In fact i was trying different ways using for example opto triac (MOC) but i also need to pick the freq of the AC network, also another based on your schematic Kiln Temperature Controller Circuit but with up down counter 4516b and 555 etc,etc so complicated

Many thanks



My Response:

Thanks dear Bernard,

The image which you had inserted in the conversation did not get attached properly and therefore it was not showing, but I have corrected it now and have posted it back in the article.

I have rated the caps at 50V because R9 is supposed to be a 33K or a 68K resistor which will drop the current significantly and not allow the capacitors to burn, this is my understanding.

I have used polarized capacitors because the gate of a triac works with a DC drive, but yes you are right, in order to make it DC for the capacitors we need to add a 1N4007 in series with the gate 1K resistors.

Now with regards to this design, if suppose the idea does not operate very smoothly or fails to produce the expected results, we could modify the existing gate drive for the 4 triacs into optocoupler based drivers, and perform the same sequential delayed switching but through an external DC circuit.So this circuit ultimately has the potentials to deliver the intended results, either this way or that way.

Feedback#2 from Mr. Bansal

I need to drive a half hp motor using an invertor of Luminous make 850 va.
On trial, the invertor trips on overload due to high starting current of  the motor. How can I bypass the overload at least during start of the  motor, or forever?


If the surge wattage is over 850VA then it can be impossible to handle this situation with the existing inverter, you may have to upgrade to a higher wattage inverter.

If the surge is within 850VA then you can probably tackle this by adding another battery only during the switch ON, and then remove the second battery once the motor has started.

Need Help? Please leave a comment, I'll get back soon with a reply!


  1. Hi

    I am having trouble getting this to work. I think that all of my wiring and components are good and the right way round, except that the impedance for the relay is 1550 ohms. What might I change to get it to work? Or any other suggestions


  2. The last circuit had issues with the diode and the switch positions, I have corrected them, it should work now.

  3. Hi Nick,

    I have answered your issue in the previous comment, the circuit needed some corrections and it's been updated in the new diagram, please check it out....your relay is OK.

  4. The circuit is actually pretty simple: If the relay contacts close, the triac would get the gate current and activate any load that's connected with its shown leads. So there's no chance the motor won't run.

    You can replace the motor with a light bulb and check the response, if still it doesn't work then surely the triac pinouts may be wrongly configured or due to some other connection fault.

  5. Hi Nick,
    Replace the capacitor across the relay coil with a 100uF/50V, this will surely rectify the issue.

  6. Hi Bro..

    How can I connect this above circuit with This one?

    i want to start a 3 phase induction motor using this circuit..

  7. I am sorry Udit, all my circuits are for single phase applications, cannot be used for 3 phase.

  8. Hi Nick,

    Yes, replace the capacitor across the relay coil with a 100uF/50 Volt.....

  9. hi..
    how are you i want to make a project to indicate the PH of water plzz can u help me.

  10. hi, if possible i'll surely try to add the design soon in my blog.

  11. Is this a good scheme for soft start:

  12. Hello again Swagatam,

    I stumbled across your blog, again, when looking for a circuit allowing me to drive an air compressor (and maybe a small fridge) from an inverter rated for 300W, with a peak of 600W. The compressor itself draws 240W, but the truck's fuse allows for 300W, no more. Direct battery connection is impossible as it sits below the driver's floor. Inrush current is unspecified.

    Do you know how much current would be drawn with your circuit when compressor is started?

  13. could be, but the last design in the above article is simpler

  14. Hello Pat,
    A soft start feature could be initiated by suitably selecting R2 in the second circuit however all the circuits explained in the above article are basically for safeguarding the concerned switches and contacts from the heavy initial surges which is implemented by diverting or sharing the current through a triac,

    Any inductive load will necessarily draw the specified amount of current and it's quite recommended, I think you can use a slow-blow fuse which would wait for a second and will not blow if the surge does not last beyond that initial phase.

    or may be you can try an electronic fuse as explained here:

  15. can you build ponticiometar to that scheme I sent you, and exactly where and how many ohm

  16. there's no easy way to insert a pot in the design, it will need to be modified greatly for that.

  17. so can you change the schema with a built-in potentiometer with labels, of course if it is not a problem for you and thank you

  18. the existing design is already set at 50% less power. for a pot control feature the MOC LED will need to be controlled through PWM using a IC555.

    If possible i'll post it in this blog soon.

  19. Hello,
    I'm not sure to properly understand what you mean by "specified amount of current". Of course I don't expect the compressor to draw anything less than the 240W written on it, just to eliminate very high inrush current to avoid burning the inverter. By adapting the last circuit to account for the necessary "kick" in motors than can't be unloaded, like in a fridge, by how much would the inrush current be reduced?

  20. Pat, If the motor is loaded, it wouldn't be correct to force a soft start to it because that would cause a choking effect on the winding and the motor would not initiate properly but keep drawing heavy current, so it has to be allowed to suck-in the required full amount of high inrush current, just to enable it to produce that initial "kick" and restore into the normal stance as soon as the motor has gained enough RPM.

    The last circuit in the above article would not stop the motor from drawing the initial high current, it just ensures that the toggle switch is kept aloof from the initial surge and the contacts never burn due to these inrush currents.

  21. Figure 3 is a not good scheme for electric motor immediately at full speed

  22. why?.....first of all it's designed for safeguarding the switch contacts not for soft start.

    however the 100uF capacitor at the triac gate could effectively produce the required soft start also...

  23. in this scheme:
    I traded a variety of values ​​of capacitors but nothing all the same moving at full speed
    and relay 12v very much to hear there are probably more than 12V

    I can not believe that the Google can not find a scheme that works soft start

  24. the motor will not take 1 second or 2 seconds to catch speed, if it does in that way it's coil could burn.

    electronics is all about understanding and then building.

    The 100 ohm resistor needs to be increased also along with the capacitor to influence a delay.

    The relay has no connection with the delay it's the triac which causes the delay.

  25. Hello, thank you for posting this circuit..

    for last circuit, Is that suitable for 100 to 300 Watt motor waterpump?
    Diode 4007 and Relay 12 V, what is that for in the circuit?


  26. Hi, thanks, yes it would be suitable since the triac is capable of handling well over 30 amps.

    the relay is for switching the triac ON, so that the switch is relieved from any sort of initial surge current and thus makes sure the switch lasts forever.

    the diode is for half-wave rectifying the relay supply voltage from the 1uF capacitor.

  27. I do understand that no more fire in pressure switch (for my case using waterpump), because TRIAC taken that job,

    But how this circuit reduce motor starting current?, As far as I can see this circuit look no delay at all...

  28. try the second circuit, the last circuit is specifically intended for protecting the contactor or the switch, it may not help in providing a slow start

  29. Hi,..

    I'm using PSpice to simulate voltage at the gate of BTA.
    I replace capacitor 100uF to 470uf, voltage increase from 0 to 5 Volt in 1 second. Is that mean Triac output will slowly increase from 1 to 220V in 1 Second?

    I Dont know the working principle of the triac,.. Sorry, I'm Newbie in Power Electronics...

  30. yes it would slow down the triac anode to cathode current but only for a very short duration, after that the triac would instantly fire to full conduction.

    You can also refer to the following design which I posted recently, and it makes more sense:

  31. Hi there, first of all I need to congratulate your writing and your site, it is very good, so far it has helped me quite a lot, that said I would like to make this circuit for my place, but I have one issue.
    I can't find this TRIAC anywhere, (there's not much supply/demand for electronic components where I live), I can find smaller TRIACS, but I don't know if they will be able to behave just like your circuit. Do you think I could replace this TRIAC with a more modest one?
    The specifications of my motor are:
    Power: 1 HP / 0.746 kW
    Voltage: 127/220 V
    Current: 16.9-8.1 A

    Any help will be highly appreciated, cheers!!

  32. Hi thanks! you could probably try using a 10amp triac which is comparatively smaller than the shown one. A 10amp triac would hopefully handle the initial kick of a 1HP motor, moreover the attached dimmer circuit would set the triac at around 25 to 50% less current so it would quite fine (the second circuit in the article)

  33. hey!! thanks a lot for the response! I managed to find a BTA16-800B, hopefully it will do the trick, I will try the second circuit in the article with this triac, and the specificacions of the motor I posted above, hopefully it will be able to have that initial kick, start, and don't have that pick of current.
    I'll post my results back, cheers!!!

  34. Hmm..BTA16-800 looks quite OK to me since it's rated to handle at least 16amps continuously.

    I sincerely hope you succeed in achieving the intended results, wish you the best!!

  35. I need a soft start for a 15 amp, 120 volt Table Saw. I think circuit 2 is what I need? what is the value of L1? I have plenty of coils and chokes but can only identify by inductance and size. My triac is rated for 40 amps.

  36. L1 is not critical, it's introduced only for suppressing noise in radio receivers, however you can make it yourself by winding around 200 turns of 1mm magnet wire over an iron bolt

  37. Any switch that can only ON then OFF using any technique without switch OFF manual.

  38. sorry, I could not understand your need....

  39. hi i want a soft starter just for 1 hp starting capacitor with clutch based motor which circuit of soft starter should i use? can u plz tell me?

  40. Hi Naveed, you can try the second circuit from the above article, I would be also publishing a PWM based soft start circuit soon, which you can refer to for your application once it gets published

  41. Hi Everyone,
    I have a small 6v dc motor to drive a model carossel I would love to have a soft start or slow start, to make it look real, max amp draw would be no more than 1 to 2 amps, nominal draw would be around 500 to 800 milliamps, does any one have any ideas as to what I can use.
    Regards Peter

  42. Hi Peter,

    you can try the following concept:

  43. Hi everyone,
    Can i have your e-mail? i want to discussion about my project. my project is water pump pressure controller. i have problem to create the circuit. tq

  44. Sir good day..sir can this ckt be applied to a three phase motor?..if not, do you have a ckt for a three phase motor

  45. Rea, for a 3 phase motor, it would be more appropriate to incorporate a PWM based soft-start, which looks reliable as described in the following article:

  46. hi sir, thanks for the blog, could u give something on disadvantages of soft starters

  47. PWM soft start probably has no disadvantages...

  48. Hello,
    Great work sir.
    Will this work for a 115v 2hp 19.8A compressor motor? I am attempting to run off of a poorly made modified-wave 5kw/10kw power inverter that cannot handle the start-up surge.
    Thanks -ray

  49. Thanks, Ray,

    yes you can try the above designs, however these will require some initial tweaking and optimizations to make it work correctly for an application.

  50. You can try a PWM based soft start circuit to tackle this, if still it does not start then the inverter power might need to be upgraded.

  51. Hi Mr. Mujamdar
    Found your website while searching for a soft start for a table-saw, very interesting site, i read all the comments for that design unfortunately i'm totally confused about which of these circuits to use.( ref:
    My table-saw is 110v 13amps brush type motor, could you pls. help choosing the appropriate circuit.

  52. Thanks Roger, the PWM approach is the most sophisticated one and according to me this concept should be ideally employed for all motors for getting a perfect soft start

    the second option from the above article is also a good one which you can try. the resistor in question will need to be optimized appropriately.

  53. Thanks for the prompt reply, i'll try the PWM option.
    Thanks again very much appreciated.

  54. I want to build your 4th circuit to drive 150W 230VAC waterpump.
    BTA16-600 is fine, right?
    Is heatsink necessary?
    Should I change 100ohm resistor? because BTA16 gate trigger current is smaller than BTA41.

    Sorry my english is bad.
    Thanks so much..

  55. yes 16 amp triac will do for a 150w motor...but heatsink will be required.

    100 ohm can be replaced with a 470 ohm resistor.

    I personally prefer the second design as it looks more effective for the proposed application.

  56. Hi Swagatam, nice blog, keep on writing. Can you please explain working of 4th(last) circuit. I am bit confused.

  57. Thanks Rahul, In the last circuit, the switch is prevented from taking the heavy load of the motor and is used to toggle the low current triac gate, this ensures a permanent life for the switch, and additionally the 100 ohm resistor along with the 100uF capacitor causers the triac to switch ON rather sluggishly which in turn enables the motor to achieve a kind of soft start...

  58. Hey Swatagam, what a great site for information. I am trying to find a circuit for a simple soft start on a single phase blower motor (120 VAC/10 amp) and am looking to limit the inrush current to 10 amps. The current inrush I'm measuring is 25 amps. I like the simplicity of the 4th circuit but am more interested in limiting the inrush current than protecting the switch. I also need something that can handle multiple on/off cycles so a NTC thermistor can't be used.

    Much Appreciated!


  59. Thanks Tim, I am glad you liked my site...yes if you are looking just to stop the inrush then an NTC would be quite good, and you can select the appropriate one as per the specifications. You can refer to this website for the chart from which you can select the one that may suit the best for your requirement

  60. sorry, I thought you preferred NTC.....however NTCs can be used for multiple ON/OFF situations also, an NTC will not get damaged until its maximum current specification is not crossed

  61. Unfortunately, the cooling time on the NTC won't work for this application and am looking for a circuit that will limit the inrush for multiple on/off cycles possibly within 10-20 seconds. So I was looking for an active circuit.


  62. In that case you can try the first circuit in this article, this circuit appears to be the most feasible one among the other designs, the relay can be eliminated if required

  63. The LED could be operated through a small capacitive power supply connected with the motor switch, and a relatively large value capacitor added across the LED for achieving a slowly increasing brightness....

  64. It is good circuit for motor soft starting.can I get complete kit from any where online purchase.

  65. Pv mM based much improved simplified circuit I wish to use along with 555 based automatic water level controller (efy circuit)with ON/OFF relay.Hope I can use your circuit.

  66. Hi Ravindra, you an surely use any of the above concepts with your existing module...

  67. Ravindra, sorry I do not have much idea regarding the availability of such kits online.

  68. Satheesh K NallusamyMarch 7, 2018 at 3:18 PM

    Swagatam sir
    magnet wire mean enamelled copper wire Right ?

  69. hi there,
    will the soft starter work for a 24V dc motor.

  70. Hi, this will not work, but you an try this:


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